1. Field of the Invention
This invention relates to an automatic focus adjusting device for use in a camera or the like.
2. Related Background Art
Many automatic focus adjusting systems in single-lens reflex cameras are such that the cycle of "focus detection (sensor signal input and calculation of focus detection) and lens driving" is repetitively effected to thereby focus a photo-taking lens on an object. The amount of lens driving in each cycle is based on the defocus amount at a point of time whereat focus detection is effected in that cycle, and this presumes that the defocus amount during focus detection is eliminated when lens driving is completed.
As a matter of course, focus detection and lens driving require their respective times, but in the case of a stationary object, the defocus amount does not vary unless the lens is driven and therefore, the defocus amount which should be eliminated at a point of time whereat lens driving is completed is equal to the defocus amount at a point of time whereat the focus is detected, and thus, proper focus adjustment is accomplished.
However, in the case of an object which is in great motion, the defocus amount may vary during focus detection and lens driving and the defocus amount which should be eliminated may differ remarkably from the detected defocus amount, which in turn may result in the problem that the lens is not in focus to the object when lens driving is completed.
As an automatic focus adjusting method directed to the solution of the above-noted problem, the assignee has proposed Japanese Patent Application No. 62-263728.
The gist of the method disclosed in this patent application is that in view of the detected defocus amount and the amount of lens driving in each said cycle and the time interval of each cycle, the relation between the imaging plane position attributable to movement of an object and time is approximated to a linear function over a quadratic function to thereby correct the amount of lens driving, and the above-noted problem can be expected to be overcome by this method.
In the case of such a foreseeing method, however, a difference between the foreseen lens position and the actual imaging plane position (a foreseeing error) results from a lens driving error and a focus detection error. This foreseeing error is usually a magnitude more times to ten and several times as great as said focus detection error and said lens driving error. Therefore, in the conventional automatic focus adjusting device, even when an object enters the depth of the imaging plane and it can be judged that the lens is in focus, if the aforedescribed foreseeing method is used, there is the possibility that the focus (imaging) position comes out of the depth of the imaging plane with a result that the photograph taken is out of focus. As an automatic focus adjusting method directed to the solution of such a problem, the assignee has proposed Japanese Patent Application No. 63-25490.
The gist of the method disclosed in this patent application is that of foreseeing functions of several orders used in the foreseeing operation, a term of high order which is ready to be affected by a focus detection error and a lens driving error and in which a great the foreseeing error occurring is corrected to thereby decrease the influence of errors occurring in a lens driving system and a focus detecting system and improve the accuracy of foreseeing.
A description will hereinafter be given of the focus deviation engendered by the above-described correction.
FIG. 2 of the accompanying drawings is a graph for illustrating the above-described lens driving correcting method. In the graph, the abscissa represents time t and the ordinate represents the imaging plane position x of an object.
Curve x(t) indicated by a solid line means the imaging plane position, at a time t, of an object coming close to the camera in the direction of the optic axis when the photo-taking lens of the camera is at infinity. Curve l(t) indicated by a broken line means the position of the photo-taking lens at the time t, and the in-focus state is provided when the curves x(t) and the l(t) coincide with each other. [t.sub.i, t.sub.i '] indicates the focus detecting operation time, and [t.sub.i ', t.sub.i+1 ] indicates the lens driving operation time. In the example shown in FIG. 2, it is assumed that the imaging plane position changes in accordance with a quadratic function (at.sup.2 +bt+c). That is, if the current and the past three imaging plane positions (t.sub.1, x.sub.1), (t.sub.2, x.sub.2) and (t.sub.3, x.sub.3) are known at a time t.sub.3, the imaging plane position x.sub.4 at a time t.sub.4 after TL (AF time lag+release time lag) from the time t.sub. 3 can be foreseen (AF time lag:the time required for focus detection and lens driving; release time lag: the time from after a release command is put out until exposure is started).
However, what can be actually detected by the camera is not the imaging plane positions x.sub.1, x.sub.2 and x.sub.3, but defocus amounts DF.sub.1, DF.sub.2, DF.sub.3 and amounts of lens driving DL.sub.1 and DL.sub.2 as converted into amounts of movement of the imaging plane. The time t.sub.4 is a future value to the last and actually is a value varying in accordance with a variation in the accumulation time of an accumulation type sensor caused by the luminance of an object or a variation in the lens driving time caused by a variation in the amount of lens driving, but here, for simplicity, it is assumed as follows: EQU t.sub.4 -t.sub.3 =TL=TM.sub.2 +(release time lag) (1)
Under the above assumption, the amount of lens driving DL.sub.3 calculated from the result of the focus detection at the time t.sub.3 can be found as follows: EQU x(t)=at.sup.2 +bt+c (2)
Considering (t.sub.1, l.sub.1) in the figure to be the origin, ##EQU1##
By substituting the equations (3), (4) and (5) for the equation (2), a, b and c are found as follows: ##EQU2##
Consequently, the amount of lens driving DL.sub.3 as converted into an amount of movement of the imaging plane at the time t.sub.4 is found as follows: ##EQU3##
A method of correcting the second-order term for reducing a foreseeing error engendered by a focus detection error and a lens driving error will now be described with reference to FIG. 3 of the accompanying drawings.
FIG. 3 shows the relation between the imaging plane position and time.
In this figure, the solid line is assumed as the imaging plane position actually moved by the movement of an object, and when errors .delta..sub.1 and .delta..sub.2 occur between the imaging plane position and the lens position at t.sub.1 and t.sub.2, respectively, the foreseeing function is as indicated by a dot-and-dash line, and the foreseeing error .delta..sub.e is about eleven times as great as .delta..sub.1 and .delta..sub.2.
So, the second-order term is corrected by a correction coefficient TF as follows when the amount of lens driving DL.sub.3 of the equation (9) as converted into the amount of movement of the imaging plane is calculated: EQU DL.sub.3 =TF.multidot.a{(TM.sub.1 +TM.sub.2 +TL).sup.2 -(TM.sub.1 +TM.sub.2).sup.2 }+b.multidot.TL+DF.sub.3 ( 10)
In the case of FIG. 3, assuming that the correction coefficient TF=0.6, the foreseeing function is as indicated by a broken line, and the foreseeing error .delta..sub.e ' decreases to about 1/8 of the uncorrected foreseeing error .delta..sub.e.
The countermeasure using such a correction has the effect of approximating a non-linear function to a linear function and therefore, is greatly effective particularly when the focus detecting operation time interval is small and the movement of the imaging plane can be approximated to a linear function.
However, when the movement of the imaging plane cannot be approximated to a linear function, a focus deviation by the correction occurs.
The occurrence of the focus deviation by the correction will now be described with reference to FIGS. 4 and 5 of the accompanying drawings.
In FIG. 4, the ordinate represents the imaging plane position and the abscissa represents time, and this figure shows a general change in the imaging plane position when an object comes close to the camera. The solid line in this figure indicates the position of the imaging plane which actually moves, and this may be approximated to a quadratic function as follows: EQU x(t)=at.sup.2 +bt+c (11)
(a&gt;0, b&gt;0)
In contrast, the function corrected by the correction coefficient TF is as follows: EQU x(t)=TF.multidot.a.multidot.t.sup.2 +b.multidot.t+c (12)
(a&gt;0, b&gt;0, 0&lt;TF&lt;1)
Here, t.sub.1 and t.sub.2 are the times when distance measurement (focus detection) was effected in the past, t.sub.3 is the current time, and t.sub.4 is the time which is the target of foreseeing. Consequently, the target at which lens driving is to be effected next is x.sub.4.
However, when the correction as represented by the equation (12) is effected, the foreseen imaging plane position at the time t.sub.4 is x.sub.4 ', and a foreseeing error (focus deviation) of .delta..sub.e occurs to in the actual value x.sub.4. This is greater as the non-linear component of the foreseeing function is greater, and becomes greater as the correction coefficient is smaller.
Here, in the case of an object which comes close, the coefficients a and b in the equations (11) and (12) are generally a&gt;0 and b&gt;0, and when objects come close at a predetermined speed, the non-linear component (here the second-order component) is greater for a near object than for a far object and the speed of movement of the imaging plane is also greater. That is, for a far object, the foreseeing error .delta..sub.e caused by the correction of the foreseeing function is sufficiently small, while for a near object, this error may pose a problem in some cases. The then focus deviation always brings about a follow-up delay, i.e., the rearward focus state, if under a general condition (a&gt;0).
In FIG. 5, the ordinate represents the imaging plane position and the abscissa represents time, and this figure shows the general movement of the imaging plane when the object goes away from the camera. In this figure, the solid line indicates the position of the imaging plane which actually moves, and this may be approximated to a quadratic function as follows: EQU x(t)=at.sup.2 +bt+c (13)
(a&gt;0, b&lt;0)
In contrast, the foreseeing function corrected by the correction coefficient TF is as follows: EQU x(t)=TF.multidot.at.sup.2 +bt+c (14)
(a&gt;0, b&lt;0, 0&lt;TF&lt;1)
Here, t.sub.1 and t.sub.2 are the times when distance measurement was effected in the past, t.sub.3 is the current time, and t.sub.4 is the time which is the target of foreseeing. Consequently, the target at which the next lens driving is to be effected is x.sub.4.
However, when the correction as represented by the equation (14) is effected, the imaging plane position at the time t.sub.4 is foreseen as x.sub.4 ' and there occurs a foreseeing error of .delta..sub.e.
Here, in the case of an object which goes away from the camera, the coefficients a and b in the equations (13) and (14) are generally a&gt;0 and b&lt;0, and in the case of objects which go away from the camera at a predetermined speed, the non-linear component (here the second-order component) is greater for a near object than for a far object, and the speed of movement of the imaging plane is also greater. That is, for a far object, the foreseeing error caused by the correction of the non-linear component of the foreseeing function is sufficiently small, while for a near object for which the non-linear component is great, this error may pose a problem in some cases. This foreseeing error always brings about the preceding of the lens, i.e., the rearward focus state (rear or far focus state), if under a general condition (a&gt;0).
Thus, when the high-order term of the foreseeing function is corrected, there has been the problem that the follow-up performance for a non-linear change in the imaging plane position is reduced and the rearward focus state is always brought about.